The iterator pattern allows you to perform some task on a sequence of items in turn. An iterator is responsible for the logic of iterating over each item and determining when the sequence has finished. When you use iterators, you don’t have to reimplement that logic yourself.
In Rust, iterators are lazy, meaning they have no effect until you call methods that consume the iterator to use it up. For example, the code in Listing 13-13 creates an iterator over the items in the vector v1
by calling the iter
method defined on Vec<T>
. This code by itself doesn’t do anything useful.
fn main() { let v1 = vec![1, 2, 3]; let v1_iter = v1.iter(); }
Once we’ve created an iterator, we can use it in a variety of ways. In Listing 3-5 in Chapter 3, we used iterators with for
loops to execute some code on each item, although we glossed over what the call to iter
did until now.
The example in Listing 13-14 separates the creation of the iterator from the use of the iterator in the for
loop. The iterator is stored in the v1_iter
variable, and no iteration takes place at that time. When the for
loop is called using the iterator in v1_iter
, each element in the iterator is used in one iteration of the loop, which prints out each value.
fn main() { let v1 = vec![1, 2, 3]; let v1_iter = v1.iter(); for val in v1_iter { println!("Got: {}", val); } }
In languages that don’t have iterators provided by their standard libraries, you would likely write this same functionality by starting a variable at index 0, using that variable to index into the vector to get a value, and incrementing the variable value in a loop until it reached the total number of items in the vector.
Iterators handle all that logic for you, cutting down on repetitive code you could potentially mess up. Iterators give you more flexibility to use the same logic with many different kinds of sequences, not just data structures you can index into, like vectors. Let’s examine how iterators do that.
Iterator
Trait and the next
MethodAll iterators implement a trait named Iterator
that is defined in the standard library. The definition of the trait looks like this:
#![allow(unused)] fn main() { pub trait Iterator { type Item; fn next(&mut self) -> Option<Self::Item>; // methods with default implementations elided } }
Notice this definition uses some new syntax: type Item
and Self::Item
, which are defining an associated type with this trait. We’ll talk about associated types in depth in Chapter 19. For now, all you need to know is that this code says implementing the Iterator
trait requires that you also define an Item
type, and this Item
type is used in the return type of the next
method. In other words, the Item
type will be the type returned from the iterator.
The Iterator
trait only requires implementors to define one method: the next
method, which returns one item of the iterator at a time wrapped in Some
and, when iteration is over, returns None
.
We can call the next
method on iterators directly; Listing 13-15 demonstrates what values are returned from repeated calls to next
on the iterator created from the vector.
Filename: src/lib.rs
#[cfg(test)] mod tests { #[test] fn iterator_demonstration() { let v1 = vec![1, 2, 3]; let mut v1_iter = v1.iter(); assert_eq!(v1_iter.next(), Some(&1)); assert_eq!(v1_iter.next(), Some(&2)); assert_eq!(v1_iter.next(), Some(&3)); assert_eq!(v1_iter.next(), None); } } fn main() {}
Note that we needed to make v1_iter
mutable: calling the next
method on an iterator changes internal state that the iterator uses to keep track of where it is in the sequence. In other words, this code consumes, or uses up, the iterator. Each call to next
eats up an item from the iterator. We didn’t need to make v1_iter
mutable when we used a for
loop because the loop took ownership of v1_iter
and made it mutable behind the scenes.
Also note that the values we get from the calls to next
are immutable references to the values in the vector. The iter
method produces an iterator over immutable references. If we want to create an iterator that takes ownership of v1
and returns owned values, we can call into_iter
instead of iter
. Similarly, if we want to iterate over mutable references, we can call iter_mut
instead of iter
.
The Iterator
trait has a number of different methods with default implementations provided by the standard library; you can find out about these methods by looking in the standard library API documentation for the Iterator
trait. Some of these methods call the next
method in their definition, which is why you’re required to implement the next
method when implementing the Iterator
trait.
Methods that call next
are called consuming adaptors, because calling them uses up the iterator. One example is the sum
method, which takes ownership of the iterator and iterates through the items by repeatedly calling next
, thus consuming the iterator. As it iterates through, it adds each item to a running total and returns the total when iteration is complete. Listing 13-16 has a test illustrating a use of the sum
method:
Filename: src/lib.rs
#[cfg(test)] mod tests { #[test] fn iterator_sum() { let v1 = vec![1, 2, 3]; let v1_iter = v1.iter(); let total: i32 = v1_iter.sum(); assert_eq!(total, 6); } } fn main() {}
We aren’t allowed to use v1_iter
after the call to sum
because sum
takes ownership of the iterator we call it on.
Other methods defined on the Iterator
trait, known as iterator adaptors, allow you to change iterators into different kinds of iterators. You can chain multiple calls to iterator adaptors to perform complex actions in a readable way. But because all iterators are lazy, you have to call one of the consuming adaptor methods to get results from calls to iterator adaptors.
Listing 13-17 shows an example of calling the iterator adaptor method map
, which takes a closure to call on each item to produce a new iterator. The closure here creates a new iterator in which each item from the vector has been incremented by 1. However, this code produces a warning:
Filename: src/main.rs
fn main() { let v1: Vec<i32> = vec![1, 2, 3]; v1.iter().map(|x| x + 1); }
The warning we get is this:
$ cargo run Compiling iterators v0.1.0 (file:///projects/iterators) warning: unused `std::iter::Map` that must be used --> src/main.rs:4:5 | 4 | v1.iter().map(|x| x + 1); | ^^^^^^^^^^^^^^^^^^^^^^^^^ | = note: `#[warn(unused_must_use)]` on by default = note: iterators are lazy and do nothing unless consumed Finished dev [unoptimized + debuginfo] target(s) in 0.47s Running `target/debug/iterators`
The code in Listing 13-17 doesn’t do anything; the closure we’ve specified never gets called. The warning reminds us why: iterator adaptors are lazy, and we need to consume the iterator here.
To fix this and consume the iterator, we’ll use the collect
method, which we used in Chapter 12 with env::args
in Listing 12-1. This method consumes the iterator and collects the resulting values into a collection data type.
In Listing 13-18, we collect the results of iterating over the iterator that’s returned from the call to map
into a vector. This vector will end up containing each item from the original vector incremented by 1.
Filename: src/main.rs
fn main() { let v1: Vec<i32> = vec![1, 2, 3]; let v2: Vec<_> = v1.iter().map(|x| x + 1).collect(); assert_eq!(v2, vec![2, 3, 4]); }
Because map
takes a closure, we can specify any operation we want to perform on each item. This is a great example of how closures let you customize some behavior while reusing the iteration behavior that the Iterator
trait provides.
Now that we’ve introduced iterators, we can demonstrate a common use of closures that capture their environment by using the filter
iterator adaptor. The filter
method on an iterator takes a closure that takes each item from the iterator and returns a Boolean. If the closure returns true
, the value will be included in the iterator produced by filter
. If the closure returns false
, the value won’t be included in the resulting iterator.
In Listing 13-19, we use filter
with a closure that captures the shoe_size
variable from its environment to iterate over a collection of Shoe
struct instances. It will return only shoes that are the specified size.
Filename: src/lib.rs
#[derive(PartialEq, Debug)] struct Shoe { size: u32, style: String, } fn shoes_in_my_size(shoes: Vec<Shoe>, shoe_size: u32) -> Vec<Shoe> { shoes.into_iter().filter(|s| s.size == shoe_size).collect() } #[cfg(test)] mod tests { use super::*; #[test] fn filters_by_size() { let shoes = vec![ Shoe { size: 10, style: String::from("sneaker"), }, Shoe { size: 13, style: String::from("sandal"), }, Shoe { size: 10, style: String::from("boot"), }, ]; let in_my_size = shoes_in_my_size(shoes, 10); assert_eq!( in_my_size, vec![ Shoe { size: 10, style: String::from("sneaker") }, Shoe { size: 10, style: String::from("boot") }, ] ); } } fn main() {}
The shoes_in_my_size
function takes ownership of a vector of shoes and a shoe size as parameters. It returns a vector containing only shoes of the specified size.
In the body of shoes_in_my_size
, we call into_iter
to create an iterator that takes ownership of the vector. Then we call filter
to adapt that iterator into a new iterator that only contains elements for which the closure returns true
.
The closure captures the shoe_size
parameter from the environment and compares the value with each shoe’s size, keeping only shoes of the size specified. Finally, calling collect
gathers the values returned by the adapted iterator into a vector that’s returned by the function.
The test shows that when we call shoes_in_my_size
, we get back only shoes that have the same size as the value we specified.
Iterator
TraitWe’ve shown that you can create an iterator by calling iter
, into_iter
, or iter_mut
on a vector. You can create iterators from the other collection types in the standard library, such as hash map. You can also create iterators that do anything you want by implementing the Iterator
trait on your own types. As previously mentioned, the only method you’re required to provide a definition for is the next
method. Once you’ve done that, you can use all other methods that have default implementations provided by the Iterator
trait!
To demonstrate, let’s create an iterator that will only ever count from 1 to 5. First, we’ll create a struct to hold some values. Then we’ll make this struct into an iterator by implementing the Iterator
trait and using the values in that implementation.
Listing 13-20 has the definition of the Counter
struct and an associated new
function to create instances of Counter
:
Filename: src/lib.rs
struct Counter { count: u32, } impl Counter { fn new() -> Counter { Counter { count: 0 } } } fn main() {}
The Counter
struct has one field named count
. This field holds a u32
value that will keep track of where we are in the process of iterating from 1 to 5. The count
field is private because we want the implementation of Counter
to manage its value. The new
function enforces the behavior of always starting new instances with a value of 0 in the count
field.
Next, we’ll implement the Iterator
trait for our Counter
type by defining the body of the next
method to specify what we want to happen when this iterator is used, as shown in Listing 13-21:
Filename: src/lib.rs
struct Counter { count: u32, } impl Counter { fn new() -> Counter { Counter { count: 0 } } } impl Iterator for Counter { type Item = u32; fn next(&mut self) -> Option<Self::Item> { if self.count < 5 { self.count += 1; Some(self.count) } else { None } } } fn main() {}
We set the associated Item
type for our iterator to u32
, meaning the iterator will return u32
values. Again, don’t worry about associated types yet, we’ll cover them in Chapter 19.
We want our iterator to add 1 to the current state, so we initialized count
to 0 so it would return 1 first. If the value of count
is less than 5, next
will increment count
and return the current value wrapped in Some
. Once count
is 5, our iterator will stop incrementing count
and always return None
.
Counter
Iterator’s next
MethodOnce we’ve implemented the Iterator
trait, we have an iterator! Listing 13-22 shows a test demonstrating that we can use the iterator functionality of our Counter
struct by calling the next
method on it directly, just as we did with the iterator created from a vector in Listing 13-15.
Filename: src/lib.rs
struct Counter { count: u32, } impl Counter { fn new() -> Counter { Counter { count: 0 } } } impl Iterator for Counter { type Item = u32; fn next(&mut self) -> Option<Self::Item> { if self.count < 5 { self.count += 1; Some(self.count) } else { None } } } #[cfg(test)] mod tests { use super::*; #[test] fn calling_next_directly() { let mut counter = Counter::new(); assert_eq!(counter.next(), Some(1)); assert_eq!(counter.next(), Some(2)); assert_eq!(counter.next(), Some(3)); assert_eq!(counter.next(), Some(4)); assert_eq!(counter.next(), Some(5)); assert_eq!(counter.next(), None); } } fn main() {}
This test creates a new Counter
instance in the counter
variable and then calls next
repeatedly, verifying that we have implemented the behavior we want this iterator to have: returning the values from 1 to 5.
Iterator
Trait MethodsWe implemented the Iterator
trait by defining the next
method, so we can now use any Iterator
trait method’s default implementations as defined in the standard library, because they all use the next
method’s functionality.
For example, if for some reason we wanted to take the values produced by an instance of Counter
, pair them with values produced by another Counter
instance after skipping the first value, multiply each pair together, keep only those results that are divisible by 3, and add all the resulting values together, we could do so, as shown in the test in Listing 13-23:
Filename: src/lib.rs
struct Counter { count: u32, } impl Counter { fn new() -> Counter { Counter { count: 0 } } } impl Iterator for Counter { type Item = u32; fn next(&mut self) -> Option<Self::Item> { if self.count < 5 { self.count += 1; Some(self.count) } else { None } } } #[cfg(test)] mod tests { use super::*; #[test] fn calling_next_directly() { let mut counter = Counter::new(); assert_eq!(counter.next(), Some(1)); assert_eq!(counter.next(), Some(2)); assert_eq!(counter.next(), Some(3)); assert_eq!(counter.next(), Some(4)); assert_eq!(counter.next(), Some(5)); assert_eq!(counter.next(), None); } #[test] fn using_other_iterator_trait_methods() { let sum: u32 = Counter::new() .zip(Counter::new().skip(1)) .map(|(a, b)| a * b) .filter(|x| x % 3 == 0) .sum(); assert_eq!(18, sum); } } fn main() {}
Note that zip
produces only four pairs; the theoretical fifth pair (5, None)
is never produced because zip
returns None
when either of its input iterators return None
.
All of these method calls are possible because we specified how the next
method works, and the standard library provides default implementations for other methods that call next
.
© 2010 The Rust Project Developers
Licensed under the Apache License, Version 2.0 or the MIT license, at your option.
https://doc.rust-lang.org/book/ch13-02-iterators.html